The invention relates to an apparatus for the forming of a strip in a continuous run on the surface of a rotating drum which is to be heated and/or to be cooled in predetermined stationary regions. The field of use is mainly the forming of plastic films or plastic sheets which are to have special optical properties due to their surface form, for example as Fresnel lenses. Another field of use is the three-dimensional patterning of wallpapers or textile materials with or without the participation of thermoplastics. The strip to be formed is brought, in the heated zone, into intimate contact with the drum surface such that it complementarily assumes the form of the latter. For example, a plastic extrusion delivered in the hot-plastic state from an extruder nozzle onto the surface of the drum is pressed by a calendar roller against the drum and fills the form depressions provided there. The strip solidifies in the following cooling zone, so that it can finally be removed in the dimensionally stable state from the drum.
In a known apparatus of this type (DE-A 19900381), that region of the drum onto which the plastic melt impinges is heated in order to prevent a premature solidification of the plastic. This is to ensure that the plastic is sufficiently flowable so that even fine form depressions of the drum surface can be filled completely. The heated region of the drum in which the plastic is delivered is followed by a cooling region in which the plastic located on the drum is cooled by cooling air from the side facing away from the drum. After its solidification, it is drawn off in strip form from the drum. Cooling from the outside has the disadvantage that it is difficult to achieve the reliable solidification also of the fine plastic structures present on the inside of the drum, although this is especially important.
It is also known (DE-A-4110248; DE-C-19943604) to cool the drum from inside while the drum surface is being heated by external heating means before the delivery region is reached. The alternating heating and cooling entail a high energy loss, because it must be presupposed that, for reasons of rigidity, the drum is thick and has a correspondingly high heat capacity. This applies all the more when the drum is heated and cooled solely from inside (U.S. Pat. No. 5,945,042, FIG. 3).
It is known, furthermore, to use an endless belt instead of a drum in order to form the plastic strip (U.S. Pat. No. 5,945,042, FIG. 1). However, deep and exact engravings, such as are required, for example, for the production of optically effective plastic moldings, cannot be joined with sufficient dimensional stability and flexural fatigue strength in endless belts.
An object is to provide an apparatus for the forming of a strip, formable at increased temperatures, in a continuous run on the surface of a rotating drum which is to be heated and/or cooled in predetermined stationary regions, in which the heat losses are reduced in spite of the use of a cylindrical drum.